The SunThe Sun’’s Energy Sources Energy Source

The first scientific theories involved chemical reactions or gravitational The first scientific theories involved chemical reactions or gravitational collapse.collapse.

• chemical burning ruled out…it can not account for the Sunchemical burning ruled out…it can not account for the Sun’’s luminositys luminosity• conversion of gravitational potential energy into heat as the Sun contracts conversion of gravitational potential energy into heat as the Sun contracts

would only keep the Sun shining for 25 million yearswould only keep the Sun shining for 25 million years• late 19late 19thth-century geological research indicated the Earth was older than that-century geological research indicated the Earth was older than that

Development of nuclear physics led to the correct answerDevelopment of nuclear physics led to the correct answer• the Sun generates energy via nuclear the Sun generates energy via nuclear fusionfusion reactions reactions• Hydrogen is converted into Helium in the SunHydrogen is converted into Helium in the Sun’’s cores core• the mass lost in this conversion is transformed into energy the mass lost in this conversion is transformed into energy • the amount of energy is given by Einsteinthe amount of energy is given by Einstein’’s equation: E = mcs equation: E = mc22

• given the Sungiven the Sun’’s mass, this will provide enough energy for the Sun to shine s mass, this will provide enough energy for the Sun to shine for 10 billion yearsfor 10 billion years

Striking a BalanceThe Sun began as a cloud of gas undergoing gravitational collapse.The Sun began as a cloud of gas undergoing gravitational collapse.

• the same heating process, once proposed to power the Sun, did cause the the same heating process, once proposed to power the Sun, did cause the core of the Sun to get hot & dense enough to start nuclear fusion reactionscore of the Sun to get hot & dense enough to start nuclear fusion reactions

Once begun, the fusion reactions generated energy which provided an Once begun, the fusion reactions generated energy which provided an outward pressure.outward pressure.

This pressure perfectly balances the This pressure perfectly balances the inward force of gravity.inward force of gravity.

• deep inside the Sun, the pressure is deep inside the Sun, the pressure is strongest where gravity is strongeststrongest where gravity is strongest

• near the surface, the pressure is near the surface, the pressure is weakest where gravity is weakestweakest where gravity is weakest

This balance is called This balance is called gravitationalgravitational equilibriumequilibrium..

• it causes the Sunit causes the Sun’’s size to remain s size to remain stablestable

Basic Properties of the SunBasic Properties of the Sun

Distance: 1.48 x 108 km = 1 A.U.

Mass: 1.99 x 1030 kg

Radius: 6.96 x 105 km

Density: 1.41 g/cm3

Luminosity: 3.8 x 1026 watts

Composition of the Sun

70%28%

0.2%0.3%

0.8%

We know this by identifying the absorption lines in the Sun’s spectrum.

These lines are formed in the photosphere.Remember, if we analyze starlight we can find:

• A star’s temperature• A star’s chemical composition• A star’s radial velocity (doppler)

Composition of the SunComposition of the Sun

CoreCore

1.5 x 101.5 x 1077 K K 0.25 R0.25 R

Radiation ZoneRadiation Zone

> 2 x 10> 2 x 1066 K K 0.70 R0.70 R

Convection ZoneConvection Zone

<< 2 x 102 x 1066 K K 0.85 R0.85 R

PhotospherePhotosphere

5.8 x 105.8 x 1033 K K 400 km thick400 km thick

ChromosphereChromosphere

1– 5 x 101– 5 x 1044 K K 2,500 km thick2,500 km thick

CoronaCorona

2 x 102 x 1066 K K 600,000 km thick600,000 km thick

Solar WindSolar Wind

> 10> 1066 K K beyond the orbit of beyond the orbit of PlutoPluto

Layers of the SunTemperature Depth

Layers of the Sun

The CoreThe Core T = 1.5 x 10T = 1.5 x 1077 K; depth = 0 K; depth = 0 – 0.25 R– 0.25 R

This is where the SunThis is where the Sun’’s energy is generated.s energy is generated.

Interior ZonesInterior Zones T < 8 x 10T < 8 x 1066 K; depth = 0.25 K; depth = 0.25 – 0.86 R– 0.86 R

Energy is transported through the interior.Energy is transported through the interior. The interior is divided into two zones:The interior is divided into two zones:

• Radiation Zone Radiation Zone • Convection ZoneConvection Zone

Boundary between them is at:Boundary between them is at:• T = 2 x 10T = 2 x 1066 K; depth = 0.70 K; depth = 0.70 RR

PhotospherePhotosphere T = 5,800 K; depth = 400 kmT = 5,800 K; depth = 400 km This is the yellow This is the yellow ““surfacesurface”” that we see. that we see.

ChromosphereChromosphere T = 1 T = 1 – 5 x 10– 5 x 1044 K; depth = 2,500 km K; depth = 2,500 km A thin layer above the photosphere where A thin layer above the photosphere where

most of the Sunmost of the Sun’’s UV light is emitted.s UV light is emitted.

courtesy of SOHO/SUMER consortium SOHO is a project of ESA and NASA

• UV image of the Sun

• light emitted from neutral Helium at 20,000 K

CoronaCorona T = 2T = 2 x 10 x 1066 K; depth K; depth 600,000 km 600,000 km The hot, ionized gas which surrounds the Sun.The hot, ionized gas which surrounds the Sun.

it emits mostly X-raysit emits mostly X-rays It can be seen in visible light during an eclipse.It can be seen in visible light during an eclipse.

X-ray image (YOHKOH telescope)

Visible image

Solar Wind

The stream of electrons, protons, Helium nuclei and The stream of electrons, protons, Helium nuclei and other ions which flow out from the Sun.other ions which flow out from the Sun.

It extends out beyond Pluto.It extends out beyond Pluto.

courtesy of SOHO consortium SOHO is a project of ESA & NASA

X-ray image of corona

UV image of solar wind

Visible image of solar wind

comet SOHO-6 (fell into Sun)

Sagittarius

Why does fusion occur in the SunWhy does fusion occur in the Sun’’s core ?s core ?

• Nuclear fusion• a reaction where heavier nuclei are

created by combining (fusing) lighter nuclei.

• all nuclei are positively charged

• Electromagnetic force causes nuclei to repel each other.

• for fusion to occur, nuclei must be moving fast enough to overcome E-M repulsion

• this requires high temperatures & pressures

When nuclei touch, the nuclear force binds them together

What is nuclear fission ?

Nuclear fission• a reaction where lighter nuclei are created

by splitting heavier nuclei.

Only fission has been achieved by mankindIt is easier to destroy than to create.

NeutronsNeutrons are not stable! They do not exist alone for long!

n p+ + e- + e (-decay)

p+ n + e+ + e(inverse -decay)

e is a neutrino ---- a weakly interacting particle which has almost no mass and travels at nearly the speed of light.

¯

Proton-Proton Chain

IN: 6 H, (2 e-) OUT: He, 2 H, 2 e, 4

Effectively 4 H nuclei are converted into 1 He nucleus and energy is released.

Why does the Sun Shine ?

But….But….

mass of He = 99.3% of 4 x mass of Hmass of He = 99.3% of 4 x mass of H

where did the .007 (4 mwhere did the .007 (4 mHH) go?) go?

into energy!!!into energy!!!

E = mcE = mc22

The Solar ThermostatThe rate of fusion reactions depends on temperature.The rate of fusion reactions depends on temperature.

• the higher the T, the faster the rate, the more energy is producedthe higher the T, the faster the rate, the more energy is produced

This fact, coupled with gravitational equilibrium, acts as a mechanism This fact, coupled with gravitational equilibrium, acts as a mechanism which regulates the Sunwhich regulates the Sun’’s energy output.s energy output.

• its energy output (luminosity) remains stableits energy output (luminosity) remains stable

The Solar LuminosityThe SunThe Sun’’s luminosity is stable over the short-term.s luminosity is stable over the short-term.

However, as more Hydrogen fuses into Helium:However, as more Hydrogen fuses into Helium:• four H nuclei convert into one He nucleusfour H nuclei convert into one He nucleus• the number of particles in Sunthe number of particles in Sun’’s core s core decreasesdecreases with time with time• the Sunthe Sun’’s core will contract, causing it to heat ups core will contract, causing it to heat up• the fusion rate will increase to balance higher gravitythe fusion rate will increase to balance higher gravity• a new a new equilibriumequilibrium is reached for stability at a higher energy output is reached for stability at a higher energy output• the Sunthe Sun’’s luminosity increases with time over the long-terms luminosity increases with time over the long-term

Models indicate the SunModels indicate the Sun’’s luminosity has increased 30% s luminosity has increased 30% since it formed 4.6 billion years ago.since it formed 4.6 billion years ago.• it has gone from 2.9 x 10it has gone from 2.9 x 102626 watts to today watts to today’’s 3.8 x 10s 3.8 x 102626 watts watts

“Observing” the Solar Interior• The SunThe Sun’’s interior is opaque…s interior is opaque…

• we can not see directly into it with lightwe can not see directly into it with light

• We can construct mathematical computer models of it.We can construct mathematical computer models of it.• the models are a grid of temperature, pressure, & density vs. depththe models are a grid of temperature, pressure, & density vs. depth• these values are calculated using known laws of physicsthese values are calculated using known laws of physics• they are tested against the Sunthey are tested against the Sun’’s observable quantitiess observable quantities

• We can directly measure sound waves We can directly measure sound waves moving through the interiormoving through the interior

• we observe we observe ““sunquakessunquakes”” in the in the photosphere by using Doppler shifts photosphere by using Doppler shifts

• motion of sound waves can be checked motion of sound waves can be checked against interior conditions predicted by against interior conditions predicted by modelsmodels

• There is another way to see directly There is another way to see directly into the core…into the core…neutrinosneutrinos!!

The Neutrino ProblemNeutrinos come to us directly from the core of the Sun, a

product of the proton-proton chain.We have detected them, proving that the theory of

nuclear fusion reactions is correct!But we only detect about 30% - 50% of the neutrinos

which are predicted by theoretic models.• either our understanding of nuclear fusion reactions or our

understanding of neutrinos is wrong!

We have since discovered three types of neutrinos:• electron (e), muon (), and tau ()• our neutrino detectors can register only electron neutrinos

If neutrinos can change type after being created, this could solve the “neutrino problem.”

Methods of Energy Transport• Radiation ZoneRadiation Zone

• energy travels as photons of light, which continually collide with particlesenergy travels as photons of light, which continually collide with particles• always changing direction (always changing direction (random walkrandom walk), photons can change wavelengths), photons can change wavelengths• this is called this is called radiative diffusionradiative diffusion

• This is a slow process!• It takes about 1

million years for energy to travel from the core to the surface.

Methods of Energy Transport• Convection zoneConvection zone

• photons arriving at bottom of convection zone are absorbed instead of photons arriving at bottom of convection zone are absorbed instead of scattered by matterscattered by matter

• the bottom of the zone is heated … hot gas rises to the topthe bottom of the zone is heated … hot gas rises to the top• cooler gas sinks to the bottom…just like when you boil a pot of water!cooler gas sinks to the bottom…just like when you boil a pot of water!• energy is brought to the surface via bulk motions of matter (energy is brought to the surface via bulk motions of matter (convectionconvection))

Photospheric Features

Granulation: the tops of convection cells seen “bubbling” on the Solar surface

Sunspots: dark spots on the surface where the temperature is cooler.

National Solar Observatory/AURA/NSFNational Solar Observatory/AURA/NSF

Sunspots

Sunspots occur in pairs; the pairs cluster into groups; and they rotate with the Sun

Sunspots come and go over an 11-year cycle.

courtesy of SOHO/MDI consortium SOHO is a project of ESA and NASA

Movie. Click to play.

What causes Sunspots?

• The answer is in their The answer is in their spectra.spectra.

• Absorption lines are split Absorption lines are split in three.in three.

• This This Zeeman EffectZeeman Effect is is evidence of strong evidence of strong magnetic fieldsmagnetic fields..

What causes a sunspot?

magnetic field slows down convection;Less heat is transported to surface; so that part of photosphere is cooler

Coronal FeaturesProminences – gas trapped in the magnetic fields is heated and elevated through the chromosphere to the corona

X-ray images from NASA’s TRACE mission.Movie. Click to launch.

Coronal FeaturesFlares – when a magnetic loop breaks, it releases matter and energy into space

X-ray images from NASA’s TRACE mission.

Movie. Click to play.

The Corona• Magnetic loops are shaken at their Magnetic loops are shaken at their

bases by turbulent motions in the bases by turbulent motions in the convection zone.convection zone.

• Kinked, twisted magnetic field Kinked, twisted magnetic field loops release energy to heat gas loops release energy to heat gas to 2 million K.to 2 million K.

• The charged gas (ions) remains The charged gas (ions) remains stuck to the magnetic loops.stuck to the magnetic loops.

• The corona is not uniform; there The corona is not uniform; there are empty patches called are empty patches called coronal coronal holesholes

• Magnetic heating explains why Magnetic heating explains why temperatures start to increase temperatures start to increase above the photosphere.above the photosphere.

courtesy of SOHO/EIT consortium SOHO is a project of ESA and NASA

Movie. Click to play.

Differential Rotation

The Sun does not rotate as a solid body (like the Earth.) The equator rotates faster than the poles.

Period of Rotation:

25 days (equator)30 days (poles)

Solar Activity• The photosphere of the Sun is covered with sunspots.The photosphere of the Sun is covered with sunspots.• Sunspots are not constant; they appear & disappear.Sunspots are not constant; they appear & disappear.• They do so in a cycle.They do so in a cycle.• It repeats every 11 yrs.It repeats every 11 yrs.

• SunSun’’s magnetic field switches polarity every 11 yrss magnetic field switches polarity every 11 yrs• so the entire cycle repeats every 22 yrsso the entire cycle repeats every 22 yrs

Sunspot Cycle

Solar Wind

• When magnetic field lines break, they release the charged particles into space.

• This so-called solar wind escapes through the coronal holes.

courtesy of SOHO/LASCO consortium SOHO is a project of ESA and NASA

Movie. Click to play.

Solar Wind

electrons, protons, He nuclei expelled by flares

Interact with Earth’s magnetic field to cause…

The Aurorae

• A strong Solar wind can A strong Solar wind can affect human affect human technology by:technology by:

• interfering with interfering with communicationscommunications

• knocking out power knocking out power gridsgrids

• damage electronics in damage electronics in space vechiclesspace vechicles

• We are not yet sure of We are not yet sure of the effect which Solar the effect which Solar activity has on Earthactivity has on Earth’’s s climate.climate.

the Northern & Southern Lights